This invention relates to servo systems for laterally positioning data heads with respect to magnetic-tape, and, more particularly, to detecting and acquiring a track following servo pattern recorded on the magnetic tape.
Magnetic tape provides a means for physically storing data which may be archived or which may be stored in storage shelves of automated data storage libraries and accessed when required. One method for maximizing the amount of data that can be stored is to maximize the number of parallel tracks on the media, and this is typically accomplished by employing servo systems which provide track following and allow the tracks to be spaced very closely.
Track following servo systems for magnetic tape data storage typically comprise a track following servo pattern of prerecorded servo tracks to allow precise positioning of a tape head which has servo sensors, with respect to the servo tracks. An example of a magnetic tape drive with a track following servo system is the IBM 3590 Magstar. The tape head comprises one or more read/write elements precisely positioned with respect to the servo sensors and which trace data tracks,parallel to the servo tracks. One example of a magnetic tape track following servo system is discussed in the incorporated U.S. Pat. No. 5,946,159, which employs magnetic tape having prerecorded servo patterns that include three parallel sets of servo edges, each servo edge being an interface between two dissimilar recorded servo signals, each set of servo edges comprising one servo edge on each of opposite lateral sides of a middle recorded servo signal. The middle recorded servo signal comprises a binary sequence of bursts, and the outer recorded servo signals comprise constant frequency recorded signal.
In the incorporated U.S. Pat. No. 5,946,159, the tape head has several spaced apart servo sensors for each servo edge, with the result that the tape head may be stepped between the servo sensors, each positioning the read/write elements at different interleaved groups of data tracks.
To further increase track density, a servo sensor may be indexed to positions laterally offset from the linear servo edges to provide further interleaved groups of data tracks. An example of a track following servo system for following an index servo position displaced with respect to an edge of a servo track is discussed in the incorporated U.S. patent application Ser. No. 09/413,327. In the example, the index positions are determined by measuring the ratio between the amplitudes of the two dissimilar recorded servo signals. Specifically, the middle recorded servo signal comprises binary bursts, and the outer servo signals comprise a constant frequency. The detected ratios, when the servo sensor senses both the outer recorded servo signal and the binary burst middle recorded servo signal, comprise ratios of the outer signal in the area where there is no binary burst, to the sum of the middle and outer signals in the area where there is a binary burst. As the servo sensor progresses from the outer signal, across the edge, to the middle servo signal, the ratios have a slope, and the index positions are positioned along the slope at specific ratios.
The tape path of tape drives such as the above IBM 3590 Magstar is a guided tape path, limiting the lateral movement of the magnetic tape. Another approach is open channel guiding in which the magnetic tape can move laterally a distance which is substantially greater than the distance between index positions and may be greater than the width of at least half of the servo pattern.
A function of magnetic tape is to maximize the data stored on the tape by insuring that there are only limited gaps between groups of data. One method of accomplishing this function is to stop the longitudinal movement of the magnetic tape and xe2x80x9cbackhitchxe2x80x9d, or reverse to beyond the end of the previous data, stop again, and accelerate up to speed in the original direction by the time that the end of the previous data is reached.
When the magnetic tape is guided in a guided tape path, the magnetic tape tends to stay in nearly the same lateral position as it is backhitched, so that the servo sensor is on the track following servo pattern at nearly the correct offset when it is accelerated up to speed in the original direction, and the servo system can move to the desired index position by the time the next data is to be written.
In open channel guiding, the lateral movement of the magnetic tape is much greater. Additionally, even with a guided path, some tapes become xe2x80x9cill behavedxe2x80x9d, and move laterally substantial distances. A complication is that the servo signals on one side of the track following servo pattern are similar to that on the other side of the pattern, differing only in the direction of slope of the ratios. Thus, a single sample will not indicate the side of track following servo pattern, and any adjustment in what is thought to be the correct direction will move the servo sensor off of the servo pattern, such that the servo system may become lost. Further, the lateral movement of the magnetic tape may be so great that the servo sensor is off of the track following servo pattern entirely when the longitudinal direction of the tape is started in the original direction. No servo position information is available once the servo sensor moves outside the pattern. An independent position sensor, such as that of U.S. Pat. No. 5,946,159, cannot be relied on to determine the correct position within a servo pattern, when such large tape movements are involved. Additionally, as tape moves to even higher track densitities, the problem of lateral movement becomes greater, even with a guided tape path.
It is an object of the present invention to detect and acquire a track following servo pattern of a magnetic tape that is subject to lateral motion.
In one embodiment, a servo system detects and acquires a track following servo pattern of a magnetic tape, wherein the magnetic tape is subject to lateral motion. The magnetic tape has a track following servo pattern of at least one set of parallel linear servo edges, each servo edge comprising an interface between two dissimilar recorded servo signals, each set of servo edges comprising an outer recorded servo signal on each of opposite lateral sides of a middle recorded servo signal.
The servo system comprises at least one servo sensor of a tape head, wherein the tape head is movable laterally of the magnetic tape. The servo sensor senses the recorded servo signals of the magnetic tape and provides sensed servo signals. A servo loop positions the tape head laterally with respect to the magnetic tape.
Servo loop logic is coupled to the servo sensor and to the servo loop. Initially, from an off-track position of the tape head servo sensor at a known side of, and displaced from, the track following servo pattern, the servo loop logic operates the servo loop to slew the tape head and thereby, the tape head servo sensor, laterally toward the track following servo pattern at a rate greater than the maximum rate of lateral movement of the magnetic tape. The servo loop logic monitors the signals of the servo sensor as the tape head servo sensor is slewed, to detect the servo signals. Upon detecting the sensed servo signals, the servo loop logic decelerates the slewing rate. Then, upon detecting that the sensed servo signals are within a lock window tolerance, the servo loop logic locks the servo loop to a position error signal representing the current lateral position of the tape head servo sensor, thereby acquiring the track following servo pattern. Lastly, upon acquiring the track following servo pattern, the servo loop logic operates the servo loop to track follow the sensed servo signals at the desired specific position error signal representing a desired displacement from a linear servo edge.
Method and tape drive embodiments of the invention are also shown.
In another embodiment, wherein the track following servo pattern comprises a middle recorded servo signal, and outer recorded servo signals, such that the sensed servo signals comprise a plateau when only an outer recorded servo signal is sensed by the tape head servo sensor, the servo loop logic, detects the sensed servo signals by detecting the plateau servo signals.
In a further embodiment, wherein the position error signals comprise detected ratios of the sensed servo signals when the servo sensor senses both the outer recorded servo signal and the middle recorded servo signal, the ratios comprising ratios of the outer signal to the sum of the middle and outer signals, the ratios having a slope; the servo loop logic, subsequent to detecting the plateau servo signals, and decelerating the slewing rate, upon detecting the slope of the determined ratios, adjusts the slew rate to a further decelerated slew rate relative to the track following servo pattern, such that the further decelerated slew rate adjusts for the lateral movement of the magnetic tape.
In a still further embodiment, wherein the servo loop logic, in locking the servo loop to the position error signal representing the current lateral position of the tape head servo sensor, monitors the determined ratios, and, upon the determined ratio equaling a threshold value indicating that the servo sensor is near the edge, locks the servo loop.
In another embodiment, the servo loop logic initially moves the servo sensor to the off-track position as determined from the independent position sensor. As above, the off-track position is at a known side of, and displaced from, the track following servo pattern.
For a fuller understanding of the present invention, reference should be made to the following detailed description taken in conjunction with the accompanying drawings.